This invention relates to a ball screw using lubricating grease and particularly to a ball screw used in a driving device for an injection molding machine or a driving device for a power press.
An ordinary ball screw is described with reference to FIG. 1. Thread grooves are formed in the outer peripheral surface of a threaded shaft 1 and the inner peripheral surface of a nut 2. They are fitted together with a plurality of balls 4 rollably held in a circulating passage comprising a helical passage 3 having both thread grooves opposed to each other, and a connecting passage connecting both ends thereof together, while lubricating the plurality of balls 4 and the circulating passage with lubricating grease so that the threaded shaft 1 and the nut 2 can move in the axial direction relative to each other while rotating about the axis.
Such a ball screw is used for a part that makes a linear movement, and serves to transmit power between the threaded shaft and the nut through the rolling and circulating balls. With such a power-transmitting ball screw, since many balls roll while also slipping, the balls and the circulating passage are subjected to stresses due to rolling friction and sliding friction simultaneously. Also, when the balls moving in the circulating passage are brought into contact with each other while turning in the same direction, since front and rear portions of the balls that are turning in directions opposite to each other are brought into contact with each other, large frictional forces are produced in such directions as to stop the rotation of the balls.
Also, with ball screws, their shape makes it difficult to accurately finish the groove shapes with required surface roughness and with the same degree of accuracy as with the inner ring and outer ring of an ordinary rolling bearing, so that the balls tend to be damaged. Thus it is considered that the durability of the balls determines the durability (or life) of the ball screw.
In order to minimize generation of a frictional force which hinders smooth movement of the balls in such a ball screw, lubricating grease is sealed around the balls and in the circulating passage of the ball screw.
Among lubricating greases used for ball screws, to a lubricating grease for a ball screw in which it is necessary to lubricate such a high-pressure frictional surface as to be used in a driving device of an injection molding machine or a driving device of a power press, an extreme-pressure agent is added. An extreme-pressure agent chemically reacts with a metallic surface which is to be a frictional surface and serves to form an extreme-pressure film that is soft and liable to be sheared and comprises an inorganic compound of iron and elements in additives, thereby preventing seizure and reducing wear.
As conventional extreme-pressure agents, sulfur compounds such as olefin sulfide, phosphorus compounds such as phosphate ester, compounds containing sulfur, phosphorus and zinc such as dialkyldithiophosphate salts and dialkyldithiocarbamate salts, molybdenum disulfide and graphite are known.
But even if a grease that is superior in wear resistance is sealed, if high loads are repeatedly applied to a ball screw, if it is used in a high-temperature atmosphere, or if it is used in an environment in which foreign matter tends to mix into the grease, the grease tends to deteriorate.
If the grease in the ball screw deteriorates, smooth rotation of the balls is impaired, so that an abnormal noise is produced or damage occurs in the balls or the circulating passage.
In order to cure such deterioration of lubricating grease, ordinarily, lubricating grease is additionally supplied to a ball screw at suitable time intervals by providing a grease-feed unit (hereinafter referred to as grease feeding)
But determining a suitable grease-feed timing or precisely determining the amount of lubricating grease to be added requires much experience. Thus it was a technique that required skill.
In particular, with ball screws used in driving portions for power injection molding machines and power presses, in which screws are driven by electric servo motors, instead of conventional hydraulic cylinders, the deteriorating speed of the grease varies according to the molding pattern. For example, the action of a cylinder for injection molding shows specific driving patterns corresponding to molded articles. Thus it is necessary to determine suitable grease-feed intervals for ball screws according to such driving patterns and adjust the grease-feed amount.
But for such adjustment of grease-feed conditions according to the use conditions of machines, many year""s experience were needed, and it was not an easy thing for a less experienced person to carry out grease feeding of a precise amount and with precise timing.
An object of the present invention is to provide a ball screw which makes it easy to precisely determine the timing of grease feeding and to correctly adjust the amount to be added.
In particular, an object of the present invention is to provide, as a ball screw for a driving portion applicable to power injection molding machines and power presses, one which makes it easy to precisely determine the timing of grease feeding and which makes the determination for correctly adjusting the added amount as easy as possible.
According to this invention, there is provided a ball screw comprising a threaded shaft formed with a thread groove in an outer peripheral surface thereof, a nut mounted on the threaded shaft and formed with a thread groove in an inner peripheral surface thereof, and a plurality of balls rollably retained in a circulating passage comprising a helical passage formed between the thread groove formed in the threaded shaft and the thread groove formed in the nut and a connecting passage connecting both ends of the helical passage, the balls and the circulating passage being lubricated by a lubricating grease, characterized in that the lubricating grease has a brightness of 1 or over.
Since such a ball screw is filled with a lubricating grease having a brightness of 1 or over, when the lubricating grease deteriorates due to mixing of worn powder after use for a prolonged time, it changes color to black and its brightness drops to less than 1, so that it is possible to easily determine the grease feed timing.
With such a ball screw, it is preferable to feed new grease upon start of change of color. By continuing such a grease feed twice or three times, it is possible to precisely determine suitable grease-feed amount and intervals.
Also for such a ball screw, it is preferable to use a lubricating grease in which a base oil having a viscosity at 100xc2x0 C. of 3.0-7.5 mm2/s is thickened by a urea-family thickener.
This is because by lowering the viscosity of the base oil so as to reach the above predetermined viscosity, lubricating grease supplied onto the frictional surface will be supplied smoothly, so that the durability of the ball screw improves. Also, if the viscosity at 100xc2x0 C. is less than 3.0, lubricating grease would be supplied excessively in a short time, which is not preferable. Also, since the flash point of the grease drops, heating and mixing during manufacture of grease become difficult.
Also, since grease thickened with a urea-family thickener has a good heat resistance, it is suitable as a ball screw used in a driving device for e.g. a power injection molding machine or in a driving device of a power press.
Also, a ball screw using a lubricating grease which contains 0.1-10 wt % of an extreme-pressure agent comprising an organic molybdenum compound but no other black-family extreme-pressure agent can be used as it is if the brightness is 1 or over. Otherwise, since it is reliably colored to a brightness of 1 or over by blending a colorant other than black, it is possible to reliably detect a state in which its color has changed black and the brightness has dropped below 1 due to deterioration of the grease. Thus it is possible to accurately determine the grease-feed time.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: